1. Field of the Invention
The present invention relates to improved gasification of solid carbonaceous materials, such as coal in a fluidized bed. More specifically, this invention relates to reduction of sulfur compounds in product gases by in situ conversion of harmful and/or unstable sulfur compounds formed during gasification to an environmentally acceptable sulfur compound within the fluidized bed and at least one selective solids withdrawal conduit. Solid carbonaceous materials provide an alternative energy source for natural gas and crude oil, but carbonaceous materials such as coal may have a relatively high sulfur content. Sulfur compounds present in the carbonaceous feed material are typically converted to harmful gaseous sulfur compounds during gasification in a fluidized bed reactor. This invention provides removal of such undesired gaseous sulfur compounds by their conversion in the fluidized bed and solids withdrawal conduit to an environmentally acceptable solid form which may be safely discharged with the spent coal solids.
2. Description of the Prior Art
Solid carbonaceous materials such as coal may be gasified by contacting with a steam and oxygen containing gas, such as air, at an elevated temperature generally in the range of about 1400.degree. to 2000.degree. F. and at elevated pressures. Products of the gasification reaction include hydrogen, carbon monoxide, carbon dioxide, hydrocarbons such as methane, and sulfur compounds such as hydrogen sulfide and carbonyl sulfide.
A preferred method for the gasification of coal is the U-GAS Process developed by Institute of Gas Technology in Chicago, Ill. (See the Oil and Gas Journal--Aug. 1, 1977, p. 51 et seq., the teachings of which are incorporated herein by reference) Basic principles and preferred embodiments of this gasification process are disclosed in U.S. Pat. No. 4,315,758, the teachings of which are incorporated herein by reference in their entirety. The U-GAS Process is capable of producing a clean, environmentally acceptable low Btu (about 150-300 Btu/SCF) fuel gas from coal. This gas can be used directly by industrial and commercial users or as a substitute for natural gas or fuel oil. In the form of synthesis gas, the products from the U-GAS Process can be used as a chemical feedstock or as a source of hot reducing gas for reducing metallic ores such as iron ore to the base metal.
The U-GAS Process, as disclosed in U.S. Pat. No. 4,315,758, overcomes problems associated with the agglomeration of ash particles within the fluidized bed gasification zone as taught by patents cited therein, but requires the removal of toxic sulfur compounds from the product gas stream. Sulfur compounds present in the carbonaceous feed material are typically converted primarily to gaseous hydrogen sulfide and carbonyl sulfide during gasification in a fluidized bed reactor. These sulfur compounds must be removed from the product gas stream before it is utilized as fuel gas, or for SNG production, ammonia synthesis, and the like to prevent the formation of toxic sulfur oxides or to avoid poisoning synthesis catalysts. One conventional method for removing sulfur compounds from coal gasification product gas requires cooling of the product gas to approximately ambient temperatures and scrubbing it with a chemical or physical absorbent. This method is undesirable from several standpoints, since it results in a significant loss in process efficiency, it requires extensive equipment, and it generates considerable waste materials.
It is known to remove sulfur oxides from flue gases produced by the combustion of coal by contacting the flue gases with limestone based materials. Finely divided limestone may be injected directly into a combustion furnace at a point somewhat removed from the flame, or particulate limestone or dolomite may be used in a fixed, moving or fluidized bed to contact and absorb sulfur oxide gases contained in a flue gas stream. It is also known that combustion of sulfur bearing coal or oil may be conducted in a fluidized bed of limestone which reacts with sulfur oxide gases produced during the combustion. Coal combustion reaction conditions differ significantly from coal or carbonaceous solids gasification reaction conditions, however, since in combustion reactors an excess of oxygen containing gas is required to promote complete combustion, while carbonaceous solids gasification is carried out under reducing conditions. Since oxidizing conditions are present in a combustion reactor, sulfur compounds present are converted to sulfur dioxide gas which reacts with limestone to produce calcium sulfate, a stable solid material which does not require further processing prior to discharge and disposal.
Introduction of alkaline earth metal oxides, such as calcined limestone or calcined dolomite with the coal feed into a coal gasifier having a flat distributor plate is taught by U.S. Pat. No. 3,969,089 and the patents cited therein to reduce the presence of sulfur compounds in coal gasification product gas. The hydrogen sulfide and carbonyl sulfide gas compounds formed during coal gasification react with the calcium oxide to produce solid calcium sulfide. Calcium sulfide is, however, unstable and decomposes under atmospheric conditions in the presence of water to produce hydrogen sulfide, a poisonous gas. The sulfurized calcined limestone is regenerated in a separate regenerator operated under oxidizing conditions to form sulfur dioxide gas and calcium oxide. This additional process step results in a loss of process efficiency, since it requires a separate reactor and transport of hot solids in water-free condition from one vessel to another and still results in sulfur oxides which are not desired to be released to the atmosphere. U.S. Pat. No. 3,970,434 teaches gasification of coal in admixture with alkaline earth metal oxides for absorption of sulfur and later combustion of the alkaline earth sulfide in a separate unit. U.S. Pat. No. 3,977,844 teaches gasification of coal in the presence of an oxide, hydroxide, bicarbonate or carbonate of an alkali or alkaline earth metal forming a metal sulfide which is transferred to a separate reactor for conversion to gaseous hydrogen sulfide which may be utilized in Claus process technology.